feat: implement the scheduler plugin

This also performs a one-time migration of the DOKKU_SCHEDULER values the scheduler plugin properties.

Closes #4739

plugins/scheduler/src/triggers/triggers.go

@@ -0,0 +1,39 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"strings"
+
+	"github.com/dokku/dokku/plugins/common"
+	"github.com/dokku/dokku/plugins/scheduler"
+)
+
+// main entrypoint to all triggers
+func main() {
+	parts := strings.Split(os.Args[0], "/")
+	trigger := parts[len(parts)-1]
+	flag.Parse()
+
+	var err error
+	switch trigger {
+	case "scheduler-detect":
+		appName := flag.Arg(0)
+		err = scheduler.TriggerSchedulerDetect(appName)
+	case "install":
+		err = scheduler.TriggerInstall()
+	case "post-delete":
+		appName := flag.Arg(0)
+		err = scheduler.TriggerPostDelete(appName)
+	case "report":
+		appName := flag.Arg(0)
+		err = scheduler.ReportSingleApp(appName, "", "")
+	default:
+		err = fmt.Errorf("Invalid plugin trigger call: %s", trigger)
+	}
+
+	if err != nil {
+		common.LogFailWithError(err)
+	}
+}